This invention relates generally to the recovery of methane from coalbed deposits. In one aspect the invention relates to a method of recovering methane from coalbed deposits using a surfactant in acid treatments.
Coal seams contain a system of vertical fractures (cleats) that are typically water saturated. The bulk of the gas contained in the coalbeds is adsorbed on the internal surface of the coalbed matrix. The recovery of the methane gas from the formation generally involves producing formation water (dewatering) from the coal seams thereby reducing the reservoir pressure. Upon reduction of the reservoir pressure, methane gas desorbs and flows through the cleat network to the wellbore.
The recovery of methane from coalbed deposits is substantially different from the recovery of hydrocarbons from sandstone or limestone type formations. The lithology and mineralogy of the coalbed deposits are substantially different from sandstone and limestone. Also, the methane hydrocarbon is deposited in coalbed by adherence to the matrix of the coal or to the surfaces of the cleats; whereas in the sandstone and limestone formations, the hydrocarbon occupies the interstices of the formation. The production mechanisms are also different: In coalbed methane production, the methane is desorbed from the coalbed, whereas in the recovery of hydrocarbons from sandstone and limestone formations, the hydrocarbon is produced by merely tapping into the formation and reducing the formation pressure which causes the pressurized gas or oil to flow into the wellbore.
As mentioned above, desorbing methane from the coalbed formation, requires dewatering the cleats to reduce the pressure to a level which permits the methane to detach from the coalbed and flow into the cleats and the wellbore. Since the cleats generally contain calcite or carbonate deposits or coal fines which restrict the flow through the cleats, it is the general practice to treat coalbed formations by acidizing to dissolve these calcareous materials. It is also common to fracture coalbed formations to interconnect the cleats with a vertical fracture and thereby provide a flow path to the wellbore.
Efforts to improve the dewatering step have involved stimulation treatments using surfactants in the well treating fluid. For example, SPE Paper 23455 describes the tendency of coalbed reservoirs to become damaged as a result of stimulation or workover treatments and discloses the benefits of treatments with certain surfactants. The paper suggests the use of an additive identified as CBMA of Dowell Schlumberger, Inc. and also discloses that other commercially available additives may be used. One is identified merely as an oil wetting surfactant commonly used in conventional oil and gas reservoirs and another as a fluorocarbon surfactant.
European Patent Application 0,444,760 A1, assigned to Dowell Schlumberger, Inc., discloses a surfactant for the dewatering of coal seams and enhancing the recovery of methane therefrom. The surfactants include (a) ethylene oxide, butylene oxide substituted alcohol, (b) organic polyethylene carbonate substituted alcohols, (c) butoxylated glycols, and (d) ethoxylated-butoxylated glycols.
Other references which disclose the use of surfactant and the removal of water from filter cakes include polyoxyethylene ethers of hexatol and hydrite partial long chain fatty acid ester such as described in U.S. Pat. No. 2,864,765, oxyalkylated surfactants described in U.S. Pat. Nos. 3,194,758, 4,156,649, and 4,206,063, dialkyl sulphosuccinates described in U.S. Pat. Nos. 4,097,390 and 4,146,473, and fluorinated alkyl quaternary ammonium iodide surfactant described in U.S. Pat. No. 4,028,257. U.S. Pat. No. 4,842,065 discloses a number of oil wetting surfactants used in sandstone or carbonate formations for producing oil.